This invention relates generally to apparatus for generating defibrillation waveforms, and more particularly to a circuit for generating a biphasic defibrillation waveform in an external defibrillator.
One of the most common and life-threatening medical conditions is ventricular fibrillation, a condition where the human heart is unable to pump the volume of blood required by the human body. The generally accepted technique for restoring a normal rhythm to a heart experiencing ventricular fibrillation is to apply a strong electric pulse to the heart using an external cardiac defibrillator. External cardiac defibrillators have been successfully used for many years in hospitals by doctors and nurses, and in the field by emergency treatment personnel, e.g., paramedics.
Conventional external cardiac defibrillators first accumulate a high-energy electric charge on an energy storage capacitor. When a switching mechanism is closed, the stored energy is transferred to a patient in the form of a large current pulse. The current pulse is applied to the patient via a pair of electrodes positioned on the patients chest. The switching mechanism used in most contemporary external defibrillators is a high-energy transfer relay. A discharge control signal causes the relay to complete an electrical circuit between the storage capacitor and a wave shaping circuit whose output is connected to the electrodes attached to the patient.
The relay used in contemporary external defibrillators has traditionally allowed a monophasic waveform to be applied to the patient. It has recently been discovered, however, that there may be certain advantages to applying a biphasic rather than a monophasic waveform to the patient. For example, preliminary research indicates that a biphasic waveform may limit the resulting heart trauma associated with the defibrillation pulse.
The American Heart Association has recommended a range of energy levels for the first three defibrillation pulses applied by an external defibrillator. The recommended energy levels are: 200 joules for a first defibrillation pulse; 200 or 300 joules for a second defibrillation pulse; and 360 joules for a third defibrillation pulse, all within a recommended variance range of no more than plus or minus 15 percent according to standards promulgated by the Association for the Advancement of Medical Instrumentation (AAMI). These high energy defibrillation pulses are required to ensure that a sufficient amount of the defibrillation pulse energy reaches the heart of the patient and is not dissipated in the chest wall of the patient.
While generating a biphasic waveform would be desirable in an external defibrillator, to date output circuits for generating a biphasic waveform have not been developed that can reliably and simply switch the higher voltages required in an external defibrillator. Some implantable defibrillators, such as those shown in U.S. Pat. Nos. 5,083,562 and 4,880,357, use a bridge circuit with multiple silicon-controlled rectifiers (SCRs) to generate a biphasic waveform. Because implantable defibrillators only apply a low energy defibrillation pulse having a maximum energy of approximately 35 joules, however, the output circuit in implantable defibrillators is not adaptable for use in the external defibrillator. A 200 joule energy pulse applied to an implantable defibrillator bridge circuit would overload the bridge circuit components and cause the circuit to fail.
In addition, although the high-energy transfer relays used in external cardiac defibrillators have performed satisfactorily, they have a variety of disadvantages. One of the major disadvantages is the electromagnetic interference (EMI) that is caused when the relay is closed. EMI can be detrimental to the signals used by nearby control circuits and makes the use of EMI-sensitive circuitry impractical during the application of the defibrillation pulse. Due to the EMI interference, external defibrillators typically temporarily place all control circuitry in an xe2x80x9cinactivexe2x80x9d state while a defibrillation pulse is applied. External defibrillators are therefore unable to verify that the switching mechanism or relay is working properly because a limited amount of circuitry is operational during the application of the defibrillation pulse.
An additional disadvantage of using a relay is that prior to the application of the defibrillation pulse, it may be impractical to test the integrity of the relay. For example, one method for testing the relay requires discharging the energy storage capacitor into a test load. This and similar methods require not only discharging most of the energy in the energy storage capacitor during each test, but also require extra circuitry including a test load.
The present invention is also directed to providing a method and apparatus that overcome the foregoing and other disadvantages. More specifically, the present invention is directed to providing a method and apparatus for verifying the integrity of an output circuit before and during the application of a defibrillation pulse.
The present invention is directed to providing apparatus that overcome the foregoing and other disadvantages. More specifically, the present invention is directed to an output circuit for an external defibrillator that is capable of applying a high-energy biphasic defibrillation pulse to a patient.
An external defibrillator having an output circuit that allows a biphasic defibrillation pulse to be discharged to a patient from an energy storage device, preferably an energy storage capacitor, is disclosed. The output circuit includes four legs arrayed in the form of an xe2x80x9cHxe2x80x9d (hereinafter the xe2x80x9cH-bridge output circuitxe2x80x9d). Each leg of the output circuit contains a solid-state switch. By selectively switching on pairs of switches in the H-bridge output circuit, a biphasic defibrillation pulse may be applied to the patient.
In accordance with one aspect of the invention, the switches in three of the legs of the H-bridge output circuit are silicon controlled rectifiers (SCRs). Preferably, only a single SCR is used in each leg. The switches in the fourth leg are insulated gate bipolar transistors (IGBTs). The use of single SCR switches simplifies the circuit as compared to the use of semiconductor modules that are large and expensive or as compared to the use of lower voltage parts which must be stacked. The use of three SCR legs further reduces the size, weight, and cost of the H-bridge output circuit in comparison with an implementation using two SCR and two IGBT legs.
In accordance with another aspect of the invention, the i-bridge output circuit is capable of conducting a biphasic waveform of 200 or more joules from the energy storage capacitor to the patient. Preferably, the H-bridge output circuit is capable of conducting a biphasic waveform equal to 360 joules, the industry standard for monophasic waveforms and the recommended level for a third defibrillation pulse by the American Heart Association. To store sufficient energy for such a biphasic defibrillation pulse, the size of the energy storage capacitor falls within a range from 15 uF to 200 uF.
Moreover, in addition to being able to conduct a high energy defibrillation pulse of 200 to 360 joules, the H-bridge output circuit is also capable of conducting a low energy defibrillation pulse for internal applications with an energy as low as 1 to 50 joules. Low energy defibrillation pulses are required when, for example, internal paddles are coupled to the defibrillator for use in surgery to directly defibrillate the heart, or for pediatric defibrillation, or for cardioversion of some arrhythmias in both pediatrics and adults. To allow the delivery of a low energy defibrillation pulse, the output circuit switches in three of the legs are driven by gate drive circuits which provide a repetitively pulsed control signal to the gates of the switches. The pulsed control signal on the gates allows the high voltage switches to remain conducting even when conducting very low currents.
In accordance with another aspect of the invention, a gate drive circuit biases on the IGBTs in the fourth leg with a sufficient voltage over a short interval to allow the leg to conduct approximately 400 amps of current without being damaged. Biasing the IGBTs in this manner allows the IGBTs to withstand a shorted discharge in the event the shock paddles are accidentally placed together, or in the event that there is a short in the circuit.
In accordance with still another aspect of the invention, all of the output circuit switches are selected to have sufficient current conducting capability to allow the switches in two of the legs on the same side of the H-bridge to provide a shorted path for the discharge of unwanted energy from the energy storage capacitor. The use of two legs on one side of the H-bridge to discharge the capacitor eliminates the need for an additional discharge circuit to perform this internal. energy dump function. In addition, the H-bridge circuit is able to perform the internal energy dump quickly and accurately using advantageous component values that would not be practical to implement in a separate discharge circuit. For example, the H-bridge circuit is able to perform an internal dump in less than one second through the use of a resistive component with a value of less than 100 ohms. Also, the internal dump may be performed using the H-bridge circuit so as to discharge only a specified amount of energy from the storage capacitor, rather than discharging the storage capacitor completely. Also, because the H-bridge circuit is used for both the internal dump and defibrillation pulse operations, the resistive component of the H-bridge circuit serves to both absorb energy during the internal dump and also to limit current during the defibrillation pulse. The resistive value is selected to be small enough to allow sufficient current to provide both an effective defibrillation pulse and a fast internal energy dump, while also being large enough to limit the current so as to protect the switches of the H-bridge circuit. The resistive component is also selected to have a high thermal capacity so that it can withstand the heat produced by the high currents that result during the H-bridge internal dump and defibrillation pulse circuit operations.
In accordance with another aspect of the invention, the resistive component of the H-bridge circuit is incorporated into a protective component that limits both current and voltage changes from the energy storage capacitor. The protective component is designed with both inductive and resistive properties. The use of a single protective component with these properties reduces the number of components that are required in the H-bridge circuit. In accordance with yet another aspect of the invention, the gate drive circuit provides a slow turn-on and fast turn-off of the IGBTs. The slow turn-on avoids jolting an electrically coupled SCR on one of the other H-bridge output circuit legs into a conducting state. The fast turn-off reduces the exposure of the IGBTs to potentially damaging high voltages that can occur across one IGBT when the other IGBT is inadvertently turned off first. The IGBT gate drive circuitry therefore reduces the size of the high-voltage parts that are necessary to protect the IGBTs.
In accordance with another aspect of the invention, an external defibrillator having an output circuit that is controlled by a microprocessor is provided. The output circuit includes several solid-state switches through which a defibrillation pulse is discharged to a patient from an energy storage device, preferably an energy storage capacitor. Prior to application of the defibrillation pulse, the integrity of each of the switches in the output circuit is verified. The integrity of the output circuit during the application of a defibrillation pulse is also verified by monitoring the changing charge level of the energy storage capacitor.
In accordance with another aspect of the invention, the output circuit is a circuit having four legs arrayed in the form of an xe2x80x9cHxe2x80x9d (hereinafter the xe2x80x9cH-bridgexe2x80x9d). Each leg of the output circuit contains a solid-state switch. By selectively switching on pairs of switches in the H-bridge, a biphasic defibrillation pulse may be applied to a patient. Prior to application of the defibrillation pulse, each of the legs in the output circuit is checked by switching the switches on in a desired order while the energy storage capacitor is partially charged.
In accordance with another aspect of the invention, the integrity of the H-bridge is monitored during application of the defibrillation pulse by periodically measuring the voltage across the energy storage capacitor. A voltage outside an expected range may indicate the failure of the H-bridge.
In accordance with still another aspect of the invention, a failed leg in the H-bridge is compensated for by identifying a pair of legs that provides a conductive path between the energy storage capacitor and the patient. If an operational pair of legs is identified, the defibrillator delivers a monophasic, rather than a biphasic, defibrillation pulse. The current or duration of the monophasic pulse may also be altered by changing the charge on the energy storage capacitor.
In accordance with yet another aspect of the invention, a scaling circuit is provided to step down the voltage across the energy storage capacitor so that it can be measured by the microprocessor. The scaling circuit is adjustable to allow the microprocessor to measure various voltage ranges across the energy storage capacitor.
In accordance with another aspect of the invention, if any error is detected before or during delivery of the defibrillation pulse, an error handling routine may be called to analyze and attempt to compensate for the indicated failure. The error handing routine generates a visual, aural, or other warning to the user to indicate that the defibrillator is not functioning properly. The warning to the user is especially advantageous under circumstances where the user might not otherwise be aware that the defibrillator is not functioning properly.
It will be appreciated that the disclosed method of testing the output circuit is advantageous in that it allows the integrity of the output circuit and connection to the patient to be checked both before and during the application of a defibrillation pulse. Providing a monophasic pulse in the present invention has a distinct advantage in that it allows a defibrillation pulse to be delivered to the patient even when part of the output circuit has failed. Moreover, the use of a scaling circuit allows the high voltages of the energy storage capacitor to be measured by the microprocessor for control of the defibrillator in real time.
It will be appreciated that the disclosed H-bridge output circuit is advantageous in that it allows either a high-energy biphasic waveform or a low-energy biphasic waveform to be generated by an external defibrillator and applied to a patient.